Evidence of genetic segregation in the apogamous fern species Cyrtomium fortunei (Dryopteridaceae)

Abstract

In apogamous ferns, all offspring from a parent are expected to be clonal. However, apogamous 'species' frequently demonstrate a large amount of morphological and genetic variations. Cyrtomium fortunei composed of four varieties (C. fortunei var. fortunei, var. clivicola, var. intermedium, and var. atropunctatum), is all reported to be apogamous triploids, but demonstrates large and continuous morphological variation. In previous studies, we showed that considerable genetic diversity was observed in many local populations of the apogamous fern 'species'. We hypothesized that genetic segregation has occurred, because neither sexual type nor intraspecific polyploidy have been observed in C. fortunei in Japan. Of 732 progeny examined (250 gametophytes and 482 sporophytes), obtained from a parental sporophyte whose pgiC genotype was estimated as aab, 11 (4.4%) gametophytes and 8 (1.7%) sporophytes showed a different genotype (aaa) from that of the parent sporophyte. We showed that genetic segregation occurs in apogamous C. fortunei in relatively high frequency. Moreover, we could first show that the segregation frequency in gametophytes is significantly higher than that in sporophytes of the next generation (χ² = 4.90, P = 0.027). It may suggest the existence of deleterious genes, which are expressed during the morphogenesis and growth of sporophytes.

The Evolutionary Dynamics of Apomixis in Ferns: A Case Study from Polystichoid Ferns

“…This problem is particularly important under the model of postorigin differentiation of apomictic taxa because the phylogenies used here are likely to underestimate the number of biological entities present. Evidence for this process was reported for the Cyrtomium fortunei complex [63]. Despite these limitations, our results are consistent with the hypotheses for the rare establishment of asexual lineages and a generally high extinction risk in these lineages [3].…”

“…Segregation in apomictic lineages was recently documented for Cyrtomium fortunei [63]. This process may also contribute to the diversity of the Cyrtogonellum clade because our results suggest the possibility of a single origin of apomixis in this lineage.…”

A current perspective on apomixis in ferns

“…The retention of functional meiosis in PE apomicts has been previously hypothesized to confer an evolutionary advantage, possibly as a means of generating genotypic diversity (DeBenedictis, ; Klekowsi, 1973). Results from Ootsuki et al () and Grusz () support this postulation, that PE could bestow long‐term benefits in apomictic ferns, whose evolutionary potential may more closely resemble a selfing mating system than the purely clonal apomixis with which they are often associated.…”

“…Their results support the claim that apomictic lineages have low levels of genotypic diversity across and, especially, within populations (e.g., in Myriopteris , Grusz & Pryer, ; and Dryopteris , Schneller & Krattinger, ; Peredo et al, ), particularly when compared to limited sampling of a single obligate outcrosser (i.e., Blechnum spicant ; Peredo et al, ). However, extensive sampling of apomictic taxa within Cyrtomium (Ootsuki et al, ) revealed comparatively high genetic diversity in these lineages across Japan. This is speculated to have resulted from niche differentiation among existing apomictic lineages and/or the frequent formation of new clones from sexual progenitors.…”

“…Using flow cytometry, they also documented viable polyhaploid (2.5x) gametophytes derived through apomixis from a 5x parent. Many other studies of apomictic ferns have likewise adopted flow cytometry (standardized with known chromosome counts) for genome size estimation and ploidy-level determination (Schneller & Krattinger, 2010;Ootsuki et al, 2011;Chao et al, 2012;Dyer et al, 2013;Chen et al, 2014;Hori et al, 2014Hori et al, , 2015. And though flow cytometry has greatly improved our understanding of these polyploid apomictic groups, recent work by Dyer et al (2013) cautions that inferring ploidy-level using correlations between nuclear DNA content and spore length should be restricted to intraspecific comparisons.…”